Rotary drills have emerged as an effective tool for specific drilling operations such as the creation of blast holes and geothermal wells. The drill typically comprises a rotary drill bit having three journal legs that mount respective cone-shaped rolling cutters via bearing assemblies that includes rollers and balls.
Typically, the drill bit is attached to one end of a drill string that is driven into the borehole via a rig. The cutting action is achieved by generating axial feed and rotational drive forces that are transmitted to the drill bit via the drill rods coupled end-to-end. Each of the cone-shaped cutters comprise externally mounted hardened cutting buttons positioned at different axial regions for optimised cutting as the drill bit rotates.
So as to cool the bearings, air is typically supplied down the drill string through the journal legs and into an internal cavity of each cutter within which the bearings are mounted. The air circulates around the bearings and is vented via the cavity mouth. Example rotating bits and cutters are described in U.S. Pat. Nos. 3,193,028; 3,921,735; 4,688,651, 4,421,184, 4,193,463; U.S. 2012/0160561; U.S. Pat. Nos. 4,390,072; 4,511,008 and SU 1357532.
In particular, the air flow to the different regions of the bearing assemblies is achieved via air flow passageways formed within a spindle (commonly referred to as a journal) that mounts each cutter and the respective bearings. Typically, the air circulates around the bearings and flows in a directional path of least resistance. Accordingly, differential cooling problems arise in existing cutting tools with certain bearing regions being inadequately cooled. As will be appreciated, insufficient air flow over the bearings leads to temperature rise due to friction and results in enhanced wear and a corresponding shortening of the operational lifetime of the bearings, the cutter and the spindle.
To prevent dust and dirt ingress into the bearing assemblies, it is known to divert a portion of the fluid (typically air) to the base region of the spindle to force and expel any debris material radially outward away from the cutter's cavity mouth positioned at the junction between the journal leg and the spindle. Example fluid directing passageways are described in U.S. Pat. Nos. 5,183,123 and 6,408,957. However, despite the supply of fluid to regions of the bearing assembly via separate distribution passageways within the spindle, existing assemblies are not optimised to provide a controlled supply of fluid being distributed effectively over all regions of the load and friction bearing surfaces whilst maintaining an exhaust flow at the cavity mouth (and possibly other regions of the cutter) to prevent debris ingress and contamination of the bearings. Accordingly, what is required is a drill tool that address the above problems.